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ovl: make private mounts longterm

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Overlayfs is using clone_private_mount() to create internal mounts for
underlying layers.  These are used for operations requiring a path, such as
dentry_open().

Since these private mounts are not in any namespace they are treated as
short term, "detached" mounts and mntput() involves taking the global
mount_lock, which can result in serious cacheline pingpong.

Make these private mounts longterm instead, which trade the penalty on
mntput() for a slightly longer shutdown time due to an added RCU grace
period when putting these mounts.

Introduce a new helper kern_unmount_many() that can take care of multiple
longterm mounts with a single RCU grace period.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
This commit is contained in:
Miklos Szeredi 2020-06-04 10:48:19 +02:00
parent b8e42a651b
commit df820f8de4
4 changed files with 31 additions and 1 deletions
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2
include/linux/mount.h
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@ -109,4 +109,6 @@ extern unsigned int sysctl_mount_max;
extern bool path_is_mountpoint(const struct path *path);
extern void kern_unmount_array(struct vfsmount *mnt[], unsigned int num);
#endif /* _LINUX_MOUNT_H */